In an electrically controlled braking system, a pedal stroke or a pedal depressing force driving brake pedal operation is detected by a sensor, and a controller controls the opening or closing of a solenoid valve or the like in accordance with a detection signal from the sensor to control a brake pressure generated by a brake pressure generator.
A variety of arrangements are known in the art for a brake pressure generator in which a brake pressure is generated as controlled by a controller. Several forms of the brake pressure generators which are commonly used in the art will be briefly described. FIG. 31 shows an example of a conventional liquid pressure braking system, a top portion of which illustrates a portion associated with rear wheels which are driving wheels, and a bottom portion of which illustrates a portion associated with front wheels which are driven wheels. When a brake pedal a is depressed, a liquid pressure is generated in a master cylinder b. For the front wheels, the liquid pressure is passed through a feed valve c to a wheel cylinder e, while for the rear wheels which are driving wheels, the liquid pressure is passed through a traction valve d and a feed valve c to a wheel cylinder e, thus actuating the brake.
The liquid pressure braking system is provided with an anti-skid braking subsystem (hereafter referred to as ABS) which regulates a brake pressure in order to prevent a tire lock during the actuation of the brake, and a traction control (hereafter referred to as TRC) which controls the brake pressure for the driving wheels in order to prevent a tire slip from occurring upon starting. If either wheel tends to be locked during the operation of the brake, such locking tendency of the wheel is removed by maintaining the wheel cylinder pressure by closing both the feed valve c and an exhaust valve f, by a decompressing control in which the feed valve c is closed while the exhaust valve f is opened to decompress the pressure, and by an intensifying control in which the feed valve c is opened while the exhaust valve f is closed to intensify the pressure. If a driving wheel tends to slip upon starting a vehicle, the tendency to slip is removed by closing the traction valve d and opening an open/close valve g to operate a pump h, thus controlling the feed valve c and the exhaust valve f associated with the wheel which tends to slip to apply a braking action to that wheel.
FIG. 32 shows an example of conventional air-over hydraulic brake, which is provided with ABS and TRC, in the similar manner as the liquid pressure braking system mentioned above. In this air-over hydraulic brake, as a brake pedal a is operated to actuate a brake valve j, an output pressure from the brake valve j is passed through an ABS modulator k to be fed to a brake actuator m for the front wheels (shown at the top portion of this Figure), while for the rear wheels which are driving wheels (shown at the bottom portion of this Figure), the output pressure from the brake valve j is passed through a double check valve n and an ABS modulator k to a brake actuator m where it is converted into a liquid pressure to be fed to a wheel cylinder p, thus actuating the brake.
If a wheel tends to be locked during the operation of the brake, the ABS modulator k is operated to remove the locking tendency of the wheel by maintaining the brake pressure, by displacing the air pressure from the brake actuator m to provide a decompression, or by feeding a pressure from an air tank q to the brake actuator m to provide an intensification. If a wheel tends to slip upon starting a vehicle, the tendency of the wheel to slip is removed by opening a traction valve r of that wheel which tends to slip to thereby allow the pressure from the air tank q to pass through the double check valve n and the ABS modulator k to be fed to the brake actuator m, thus activating the brake. The pressure fed to the brake actuator m is controlled by the ABS modulator k.
FIG. 33 shows an example of a conventional full air brake system. During a normal brake operation, as a driver operates a brake pedal a to operate a brake valve j, an output pressure from the brake valve j is passed through an ABS modulator k to be fed to a brake cylinder s, thus activating the brake for the front wheels, which are shown in the top portion of this Figure. For the rear wheels, the output pressure from the brake valve j is supplied as an indicator pressure for a relay valve t, which regulates an air pressure from a tank q in accordance with the indicator pressure to be fed through an ABS modulator k to a brake cylinder s, thus actuating the brake.
The full air brake of this example is provided with ABS, whereby if a wheel tends to lock during the operation of the brake, the ABS modulator k operates to remove the locking tendency of the wheel by maintaining the brake pressure, or providing a decompressing or intensifying control.
FIG. 34 shows an example of a conventional electrically controlled braking system, as disclosed in Japanese Laid Open Patent Application No. 155,162/1987, and which is provided with a full air brake pressure generator. An air tank q is connected to a brake cylinder s through a pressure control valve u, and when a brake pedal a is operated, a pressure transducer v connected to the brake pedal a converts the position of the brake pedal a into an electrical brake pressure signal, which is input to a controller w. The controller w switches the pressure control valve u in accordance with this signal, whereby an air pressure is fed from the air tank q to the brake cylinder s, thus generating a brake pressure.
It is recognized that there are three factors which determine a brake response, namely, a pedal depressing force, a pedal stroke and a deceleration (or brake pressure), a combination of which determines a brake feeling. To provide an optimum brake feeling, it is desirable if a relationship between a pedal depressing force and a pedal stroke and a relationship between a pedal depressing force or pedal stroke and a brake pressure, be optimally controlled.
However, in a conventional electrically controlled braking system, there is provided a stroke sensor which detects a pedal stroke or a depressing force sensor which detects a pedal depressing force so that the brake pressure can be controlled in accordance with either the pedal stroke or the brake depressing force. However, it has been impossible to control a relationship between the pedal stroke and the pedal depressing force, and accordingly, it has been difficult to provide a most preferred brake feeling.